1. Field of the Invention
The present invention generally relates to a method of preparing a thermoelectric material, a method of forming a thermoelectric device, and a method of fabricating a thermoelectric module. More specifically, the present invention relates to a method of preparing a thermoelectric material by using a melting-solidification method, wherein the thermoelectric material is suitable for a thermoelectric device such as a Peltier device. Furthermore, the present invention relates to a method of forming a thermoelectric device that includes the thermoelectric material by using the melting-solidification method. Moreover, the present invention relates to a method of fabricating a thermoelectric module that includes the thermoelectric material by using the melting-solidification method.
Priority is claimed on Japanese Patent Application No. 2005-245022, filed Aug. 25, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
A thermoelectric material can be used for a variety of thermoelectric devices such as the Peltier device. Typical examples of preparing the thermoelectric material may include, but are not limited to, a unidirectional solidification method, a hot-pressing method, and a deformation processing method.
In accordance with the unidirectional solidification method for preparing the thermoelectric material, a raw material is weighed. The weighed raw material is then melted to prepare a melt of raw material. This melt of raw material is then slowly cooled and solidified, while the melt of raw material has a thermal gradient. The solidified material has a unidirectionally oriented crystal structure. The solidified material is used as a thermoelectric material. The thermoelectric material prepared by the unidirectional solidification method is likely to have a high power factor (P.F.). The unidirectional solidification method is advantageous in low manufacturing cost. The power factor (P.F.) is an output factor that is expressed by Seebeck coefficient and resistivity. The power factor (P.F.) is given by the following equation.(P.F.)=α2/ρwhere α(V/K) is the Seebeck coefficient, and ρ(Ωm) is the resistivity.
In accordance with the hot-pressing method for preparing the thermoelectric material, a raw material is weighed. The weighed raw material is then melted to prepare a melt of raw material. This melt of raw material is then solidified to obtain an ingot of raw material. The ingot of raw material is then ground or milled to obtain powders of the raw material. The powders of the raw material are then filled into a cavity of a mold. Alternatively, the melt of raw material can be rapidly cooled to form powders or flakes of the raw material. The powders or flakes of the raw material can be then filled into the cavity of the mold. The powders or flakes of the raw materials are then pressed and sintered in the cavity of the mold to obtain a thermoelectric material that has a high strength. The thermoelectric material is suitable to be processed for a variety of thermoelectric device.
In accordance with the deformation processing method for preparing the thermoelectric material, a raw material is weighed. The weighed raw material is then melted to prepare a melt of raw material. This melt of raw material is then solidified to obtain an ingot of raw material. The ingot of raw material is then ground or milled to obtain powders of the raw material. Alternatively, the melt of raw material can be rapidly cooled to form powders or flakes of the raw materials. The powders or flakes of raw material are then hot-extruded. The hot-extruded raw material is then deformed by a forging method or an ECAP (Equal-Channel Angular Pressing) method, thereby obtaining a thermoelectric material that has a high strength. The thermoelectric material is suitable to be processed for a variety of thermoelectric device. Further, the thermoelectric material can exhibit high thermoelectric performance.
The thermoelectric material prepared by the above-described conventional method can be used to form a thermoelectric device. The thermoelectric device can further be used to fabricate a thermoelectric module. FIGS. 10A through 10E are perspective views illustrating sequential steps involved in a conventional method of fabricating a thermoelectric module.
The above-described conventional method is used to prepare an ingot of thermoelectric material 101, for example, a p-type thermoelectric material and an n-type thermoelectric material. FIG. 10A illustrates an ingot of thermoelectric material 101. The ingot of thermoelectric material 101 is sliced into plural wafers of thermoelectric material 102 by using a wire saw or an inside blade cutter.
With reference to FIG. 10B, plated layers 103 are formed on both surfaces of the thermoelectric material wafer 102. The plated layers 103 perform as barrier layers. The plated layers 103 can be made of a metal such as Ni.
With reference to FIG. 10C, the thermoelectric material wafer 102 with the plated layers 103 is diced into a plurality of thermoelectric devices 104 that are shown in FIG. 10D. For examples, a plurality of p-type thermoelectric devices is formed from the p-type thermoelectric material. Another plurality of n-type thermoelectric devices is formed from the n-type thermoelectric material. Each of the thermoelectric devices 104 has a cubic shape or a rectangular parallelepiped shape.
With reference to FIG. 10E, top and bottom substrates 106 and 105 are prepared, each of which has an array of electrodes. The plurality of thermoelectric devices 104 are mounted on the bottom substrate 105 so that the p-type and n-type thermoelectric devices are electrically connected in series to each other. The bottom substrate 105 has an array of the thermoelectric devices 104. The top substrate 106 is then bonded to the array of the thermoelectric devices 104 that is mounted on the bottom substrate 105, thereby fabricating a thermoelectric module. The bonding process can be carried out by using a soldering process in combination with a reflow apparatus or a hot plate.
Japanese Unexamined Patent Application, First Publication, No. 8-228027 discloses another conventional method of forming a thermoelectric device. A mold having a plurality of holes is dipped into a melt of thermoelectric material so that the plurality of holes is filled with the melt of thermoelectric material. The mold is cooled unidirectionally from one side thereof, so that the melt of thermoelectric material in each of the holes is unidirectionally solidified while the melt of thermoelectric material has a thermal gradient, for example, in the range of approximately 20° C./cm to approximately 40° C./cm. The unidirectional solidification forms a single crystal of thermoelectric material in each of the holes. The single crystal of thermoelectric material is defined by the shape of each of the holes. The single crystal of thermoelectric material has a bar shape. Namely, an ingot of single crystal thermoelectric material is formed in each of the holes. Each ingot of the single crystal of thermoelectric material is cut and divided into plural chips of crystal thermoelectric material at a predetermined length or size, thereby forming a plurality of thermoelectric devices.
Japanese Unexamined Patent Application, First Publication, No. 2003-347608 discloses another conventional method of forming a crystal of thermoelectric material for thermoelectric device. A mold release agent is applied on cavity walls of the mold. The mold release agent has a main component of carbon. The cavity of the mold has a cross-sectional area of at most 10 mm2 and a length of at least 10 mm. A melt of thermoelectric material is flown into the cavity that is coated with the mold release agent. A crystallization of the melt of thermoelectric material is carried out at a rate of at most 2 mm/hour to form a crystal of thermoelectric material. The crystal of thermoelectric material is annealed at a temperature in the range of 80° C. to approximately 400° C. This conventional method improves the yield and reduces the manufacturing cost.
Japanese Unexamined Patent Application, First Publication, No. 2004-63768 discloses still another conventional method of preparing a thermoelectric material. A melt of thermoelectric material is crystallized to form a crystal ingot of thermoelectric material by using Bridgman method, Czochralski method, or zone-melt method. The crystal of thermoelectric material has crystal grain boundaries, on which an additive element is segregated and deposited. The crystal ingot of thermoelectric material is then exposed to a heat treatment in a vacuum or an inert gas so as to diffuse the additive element from the boundaries of crystal gains into the inside of crystal grains.
The above-described unidirectional solidification method can produce or prepare a thermoelectric material at a low cost, the thermoelectric material being likely to have a high performance. The prepared thermoelectric material has a cleavage. The cleavage makes it difficult to process the thermoelectric material to form a thermoelectric device. This issues can be solved by using a mold in solidifying the melt of thermoelectric material as disclosed in the above publications, for example, Japanese Unexamined Patent Applications, First Publications, No. 8-228027 and No. 2003-347608. In accordance with those conventional methods, the rod-shaped ingot of crystal thermoelectric material is cut or divided into chips of crystal thermoelectric material, thereby forming a plurality of thermoelectric devices. Plated layers are selectively formed on opposite surfaces of each of the thermoelectric devices. The selective formation of the plated layers is not easy.
The unidirectional solidification method further includes a cooling process that is carried out at a slow cooling rate. Cooling process at a slow cooling rate forms coarse crystal grains in the crystal thermoelectric material. The crystal thermoelectric material including coarse crystal grains is brittle. Thus, the thermoelectric device is also brittle. Whereas Japanese Unexamined Patent Application, First Publication, No. 8-228027 discloses that the melt of thermoelectric material in the mold can be rapidly cooled. The cooling direction is parallel to a longitudinal direction of the mold. Practically, however, it is not effective to carry out a unidirectional rapid cooling process in the cooling direction parallel to the longitudinal direction of the mold that contains the melt of thermoelectric material.
In accordance with the above-described conventional method disclosed in Japanese Unexamined Patent Application, First Publication, No. 2004-63768, the melt of thermoelectric material is crystallized to form a crystal ingot of thermoelectric material by using Bridgman method, Czochralski method, or zone-melt method. The crystal of thermoelectric material is then exposed to the heat treatment in the vacuum or the inert gas, while an additive element is segregated and deposited on the crystal grain boundaries of the crystal ingot of thermoelectric material. Segregation and deposition of the additive element make it difficult to obtain a uniform distribution of physical property of the thermoelectric material.
The thermoelectric material prepared by the hot-pressing method has a high strength and is suitable to be processed for a variety of thermoelectric device. The thermoelectric material is engaged with disadvantages in low thermoelectric performance and high manufacturing cost.
The thermoelectric material prepared by the deformation processing method has a high strength and is suitable to be processed for a variety of thermoelectric device. The thermoelectric material is likely to exhibit a high thermoelectric performance. The thermoelectric material is, however, disadvantageous in extremely high manufacturing cost.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exist needs for an improved method of preparing a thermoelectric material. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.